The invention relates to a constant current source with a first and a second controlled current source, a first, a second and a third transistor, and a first resistor, where the base of the second transistor is connected to the collector of the first transistor, the emitter of the first transistor is at the reference potential of the circuit, the first controlled current source is connected to the base of the first transistor, the second controlled current source is connected to the collector of the second transistor, which in turn is connected to the base of the third transistor, and where either the first resistor connects the base of the first transistor with its collector and the emitter of the second transistor is at reference potential or the first resistor connects the emitter of the second transistor with the reference potential and the base of the first transistor is connected directly to its collector. A constant current source of this kind is known from DE 36 10 158.
The constant current source described in DE 36 10 158, especially FIG. 4, includes a band-gap reference voltage source with which a constant current is generated. A band-gap circuit of this kind is generally made in integrated-circuit technology supplying an output voltage that remains substantially constant with variations in temperature. Use is made here of a specific transistor base-emitter voltage V.sub.BE characteristic that varies with temperature by employing a first transistor operated as diode and a second transistor with different emitter current intensities in order to create a voltage across a first resistor that is proportional to the difference between the respective base/emitter voltages V.sub.BE. This differential voltage has a positive temperature coefficient and consequently also the current flowing through the first resistor. With the help of a current balancing circuit, this current is copied into an output circuit. In DE 36 10 158 it is demonstrated how the temperature coefficient of this output current can be brought to zero by one or two additional resistors.
The generation of a temperature-independent output voltage is also known, by conducting the current through a series circuit comprising a diode and a resistor.
The above-mentioned known constant current source includes a starting circuit which activates and keeps active the constant current source when a supply voltage is applied.
With regard to a low power consumption, it is useful in some applications to be able to switch off the constant current source when required.